<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>初始包含块</title>
  <style>
    
    *{
      margin: 0;
      padding: 0;
    }
    
    html{
      width: 500px;
      height: 500px;
      margin-left: 100px;
      border: 2px solid pink;
    }
    body{
      width: 300px;
      height: 300px;
      margin-left: 50px;
      border: 2px solid black;
    }
    #box{
      position: absolute;
      left: 0;
      top: 0;
      width: 100px;
      height: 100px;
      background-color: purple;
    }
  </style>
</head>
<body>
  <div id="box"></div>
</body>
</html>